1. Technical Field
This invention relates to cell and method for the electrolysis of a compound and to the production of a metal such as aluminum by electrolysis of a compound of the metal such as alumina in a molten electrolyte such as cryolite.
2. Description of Conventional Art
Electrolysis involves an electrochemical oxidation-reduction associated with the decomposition of a compound. An electrical current passes between two electrodes and through an electrolyte, which can be the compound alone, e.g., sodium chloride, or the compound dissolved in a liquid solvent, e.g., alumina dissolved in cryolite, such that a metallic constituent of the compound is reduced together with a correspondent oxidation reaction. The current is passed between the electrodes from an anode to a cathode to provide electrons at a requisite electromotive force to reduce the metallic constituent which usually is the desired electrolytic product, such as in the electrolytic smelting of metals. The electrical energy expended to produce the desired reaction depends on the nature of the compound and the composition of the electrolyte. However, in practical application, the cell power efficiency of a particular electrolytic cell design can result in wasted energy depending on factors such as, inter alia, cell voltage and current efficiency.
Much of the voltage drop through an electrolytic cell occurs in the electrolyte and is attributable to electrical resistance of the electrolyte, or electrolytic bath, across the anode-cathode distance. The bath electrical resistance or voltage drop in conventional Hall-Heroult cells for the electrolytic reduction of aluminum from alumina dissolved in a molten cryolite bath includes a decomposition potential, i.e., energy in aluminum product, and an additional voltage attributable to heat energy generated in the inter-electrode spacing by the bath resistance, which heat energy generally is discarded. Such discarded heat energy typically makes up 35 to 45 percent of the total voltage drop across the cell, and in comparative measure, as much as up to twice the voltage drop attributable to decomposition potential. Reducing the anode-cathode separation distance is one way to decrease this energy loss.
However, whenever the anode-cathode distance is reduced, short circuiting of the anode and cathode must be prevented. In a conventional Hall-Heroult cell using carbon anodes held close to, but separated from, a metal pad, this shorting is caused by an induced displacement of the metal in the pad. Such displacement can be caused in large part by the considerable magnetic forces associated with the electrical currents employed in the electrolysis. For example, magnetic field strengths of 150 gauss can be present in modern Hall-Heroult cells. This metal displacement can take the form of (1) a vertical, static displacement in the pad, resulting in an uneven pad surface such that the pad has a greater depth in the center of the cell by as much as 5 cm; (2) a wave-like change in metal depth, circling the cell with a frequency of on the order of 1 cycle/30 seconds; and (3) a metal flow with flow rates of 10-20 cm/second being common. Thus, to prevent shorting, the anode-cathode separation must always be slightly greater than the peak height of the displaced molten product in the cell. In the case of aluminum production from alumina dissolved in cryolite in a conventional Hall-Heroult cell, such anode-cathode separation is held to a minimum distance, e.g., of 4.0-4.5 cm.
Another adverse result from reducing anode-cathode distance is a significant reduction in current efficiency of the cell when the metal produced by electrolysis at the cathode is oxidized by contact with the anode product. For example, in the electrolysis of aluminum from alumina dissolved in cryolite, aluminum metal produced at the cathode can be oxidized readily back to alumina or aluminum salt by a close proximity to the anodically produced carbon oxide. A reduction in the anode-cathode separation distance provides more contact between anode product and cathode product and significantly accelerates the reoxidation of reduced metal, thereby decreasing current efficiency.
A consumable anode, such as the carbon anode conventionally used in the production of aluminum in a conventional Hall-Heroult cell, presents a substantial obstacle to achieving a precise control of inter-electrode spacing. In the conventional Hall-Heroult cell, oxygen gas produced at the anode combines with the carbon of the anode itself to form a carbon oxide, such as carbon monoxide and carbon dioxide gas. Oxidation of the anodes according to the overall reaction EQU Al.sub.2 O.sub.3 +3/2C.fwdarw.2Al+3/2CO.sub.2,
together with air burning of the anodes, consumes about 0.45 pounds of carbon for each pound of aluminum produced. This carbon loss in well-designed cells is largely offset by metal accumulation in the metal pad cathode of the Hall-Heroult cell, theoretically maintaining electrode spacing. However, in a cell with multiple carbon anodes, each has unique electrical properties and will have a different stage of consumption. For a number of such practical considerations, anode height must be monitored and adjusted frequently in conventional Hall-Heroult cell practice.
One direction taken to overcome the problem of anode consumption is disclosed in Haupin, U.S. Pat. No. 3,755,099, and related patents, such as U.S. Pat. No. 3,822,195, U.S. Pat. No. 4,110,178, U.S. Pat. No. 4,140,594, U.S. Pat. No. 4,179,345, and U.S. Pat. No. 4,308,113, which involve the production of a metal such as aluminum or magnesium electrolytically from the metal chloride dissolved in a molten halide of higher decomposition potential. Since an oxygen species is absent, the problem of oxygen gas combining with carbon anodes is avoided. In the absence of oxygen, carbon electrodes can be stacked one above the other in a spaced relationship established by interposed refractory pillars, as shown in FIG. 1 of U.S. Pat. No. 3,755,099. The pillars are sized to space the electrodes closely as for example by less than 3/4 inch (1.91 cm). The electrodes depicted in the figures of the above-referenced patents are shown to be rigidly supported horizontally by the wall of the cell.
Another direction is DeVarda, U.S. Pat. No. 3,554,893, which shows an electrolytic furnace having carbon electrodes that do not contact the floor or wall of the furnace. Spacers, e.g., of electrically insulating refractory material, separate the electrodes against an upward thrust exerted upon them by the bath (the bath density being higher than that of the carbon). The spacers are not attached to any electrode but rather are held in place by the upward thrust of the bath acting upon the more buoyant graphite. In Devarda, the carbon electrodes are used in the electrolytic decomposition of alumina dissolved in a bath of cryolite and thereby are consumed at the anodic portions.
DeVarda employs an inter-electrode zone similar to a conventional Hall-Heroult cell, i.e., a large anode-cathode separation between the metal pad on the base of the cell and the last or lower carbon electrode. DeVarda employs cathodes consisting of metal pad, which represents a further similarity to the Hall-Heroult process. In another aspect, it would appear that the electrodes shown in DeVarda would sink at some point when enough carbon is consumed and sufficient metal builds up in the concave cathode reservoir to exceed a reduced buoyancy of the consumed electrode.
Jacobs, U.S. Pat. No. 3,785,941, like Haupin and others discussed above, relates to chloride electrolysis. This patent discloses that the aluminum chloride-containing electrolyte tends to react with conventional refractory materials. Nitride-based refractory material is applied, e.g., as material for a spacer between the anode and cathode, in order to overcome this problem. Jacobs shows the cathode supported by the cell floor.
Alder, U.S. Pat. No. 3,930,967, shows the production of aluminum from aluminum oxide where electric power is passed through a multi-cell furnace with at least one inconsumable bipolar electrode, including an anode of a ceramic oxide. The interpolar distance is held constant by electrodes which are rigidly fixed to the floor or wall of the cell.
Foster, U.S. Pat. No. 4,297,180, shows the use of a cathode grate or hollow body for protruding the cathode surface toward the anode and above the liquid pad formed on the cell bottom. The cathode elements are shown to be supported by the floor of the cell.
Cohen, U.S. Pat. No. 4,288,309, discloses the use of consumable electrodes and spacing between two consecutive electrodes, which spacing nevertheless remains constant irrespective of the degree of erosion of the consumable electrodes. Spacer elements, having the same thickness and shaped in the form of balls, are threaded on vertical wires attached to horizontal bars associated with the top portion of the tank. The Cohen patent mentions electrolysis of liquid solutions such as sea water. Cohen does not appear to use a liquid pad of electrolytic product separate from the electrolyte.
Vertical electrodes are well known in electrolysis processes and were shown as early as Hall, U.S. Pat. No. 400,664. The Hall process disclosed therein avoided contacting the electrodes with the liquid aluminum product when the electrode was not an integral part of the internal cell surface. Alder, U.S. Pat. No. 3,930,967, shows an example of vertical bipolar electrodes, which as discussed hereinbefore are rigidly fixed to the floor or wall of the cell.
Ransley, U.S. Pat. No. 3,215,615, shows an example of inclined monopolar electrodes for producing aluminum at inclined cathodes which are rigidly fixed in the internal floor surface of the cell. The inclined anode is a consumable anode and is shown having a conical profile.
DeVarda, U.S. Pat. No. 3,730,859, is illustrative of a bipolar electrode assembly having inclined surfaces. DeVarda, U.S. Pat. No. 3,730,859 does not disclose the manner of supporting electrodes in the cell. Further, DeVarda, U.S. Pat. No. 3,730,859 discloses electrically connecting the cathode to a power supply not through the liquid metal pad but rather through current-supply connecting bars external to the cell.